1. Field of the Invention
The present invention relates to an image pickup lens system. More particularly the invention relates to the high-performance and compact image pickup lens system suitable for digital input equipment such as a digital still camera and a digital video camera which captures an image of a subject with a solid-state image pickup device.
2. Description of the Related Art
Recently, as personal computers become widespread, digital still cameras and digital video cameras (hereinafter referred simply to as “digital camera”) which can easily capture image information in the digital equipment become widespread in a personal user level. It is expected that such digital cameras become wide spread more and more in the future as input equipments for image information.
Miniaturization of a solid-state image pickup device such as CCD (Charge Coupled Device) which is mounted on the digital camera is also proceeding, and further miniaturization of the digital camera is required as the solid-state image pickup device is miniaturized. Therefore, the miniaturization is also strongly demanded in the image pickup lens system which occupies the largest capacity in the digital input equipment. In order to miniaturize the image pickup lens system, the easiest method is to decrease a size of the solid-state image pickup device. However, in order to decrease a size of the solid-state image pickup device, it is necessary to decrease the size of a photodetector, so that performance required for the image pickup lens system is increased while the degree of difficulty in producing the solid-state image pickup device.
On the other hand, when the size of the image pickup lens system is decreased while the size of the solid-state image pickup device remains, an exit pupil position is necessarily caused to be near to an image surface. When the exit pupil position is caused to be near to the image surface, an off-axis luminous flux outgoing from the image pickup lens system is obliquely incident to the image surface, so that a micro lens provided in front of the solid-state image pickup device does not sufficiently exert focusing performance. This results in a problem that brightness of the image is changed between a central portion of the image and a peripheral portion of the image. When the exit pupil position of the image pickup lens system is kept far away from the image surface in order to solve the problem, upsizing of the whole image pickup lens system can not be avoided.
Because of low-price competition in recent years, cost reduction is also strongly demanded of the image pickup lens system. Further, the higher performance is also required for the image pickup lens system due to recent integration of the solid-state image pickup device. For the above-described demands, the image pickup lens system for the solid-state image pickup device having a four-lens configuration has been proposed in Patent References (U.S. Pat. No. 6,414,802, US2003/117,721, and US2004/125,468).
However, the image pickup lens systems described in these Patent References include four lenses and the first lens is the negative lens, so that a total length of the lens is increased and the image pickup lens system lacks compactness.